Compact discs have main information such as music information which is recorded in the form of a sequence of pits having lengths in the range from 3T to 11T where T represents the basic period of a data sequence of the main information, by converting the main information into a digital signal, processing the signal according to an error correcting encoding process, modulating the signal according to an EFM (Eight to Fourteen Modulation) process, and controlling a light beam with the EFM-modulated signal to burn the pits.
Compact discs have a lead-in area in their inner circumferential region for recording managerial data called TOC (Table of Contents). Based on the TOC data, the user can select and play desired ones of music pieces that are recorded on a compact disc.
Compact discs also have a code engraved inwardly of the lead-in area as representing the manufacturer, the manufacturing factory, the disc number, etc. for the user to visually confirm the record of a compact disc.
Since the user can visually confirm the record of a compact disc from an engraved code on the compact disc, the user can identify the compact disc as an illegal copy based on whether the compact disc carries an engraved code or not. However, as the engraved code is primarily intended to give the user a visual confirmation, it is difficult for the optical pickups of some compact disc players to reproduce the engraved code. For identification of an illegal copy based on an engraved code, therefore, it is necessary to use a dedicated reproducing mechanism for reproducing engraved codes.
If auxiliary information that can be reproduced by an optical pickup for reproducing audio data can be recorded on compact discs without affecting the reproduction of audio data from a sequence of pits on the compact discs, then illegal copies can be rejected using the auxiliary information without the need for a dedicated reproducing function to reproduce engraved codes.
The applicant has previously presented an apparatus for recording auxiliary information on a compact disc by changing the width of a portion of pits or marks having a predetermined length or more, of all pits or marks formed on an optical disc, based on the data sequence of the auxiliary information, as disclosed in Japanese Patent No. 3,292,295.
The disclosed apparatus according to Japanese Patent No. 3,292,295 records, as auxiliary information, identification data for identifying the optical disc or key information for decrypting encrypted main information recorded on the optical disc. The disclosed apparatus employs a pseudo-random number sequence, typified by M-sequence random numbers, to record auxiliary information for preventing the recorded auxiliary information from being easily read for illegal copies.